Bile salts are known to play a significant role in the solubility of dietary lipids and related fat-soluble substances in the upper intestine. This solubilization by micellular aggregates of the bile salts is a direct aid in the absorption of these materials into the general circulation. Bile salt micelles have also been found to participate very directly in the function of enzymes, such as lipase. The objective of this proposed research is to study the binding of fatty acids and other lipidic materials to the bile salt micelles. Calorimetry methods will be used to evaluate binding constants, enthalpies, entropies, and heat capacities of the binding reactions. In addition, direct measurements of the heat capacities of the micelles and micellular complexes will be made. In the end, attempts will be made to find models for the binding reactions which are based on the thermodynamic behavior of simple compounds. The models will provide a basis for understanding the structural features of the substrates and bile salts and solution conditions necessary for solubilization and enzyme function of the bile salts.